This article assesses the performance and reduction level of five engine oils available from various manufacturers. The trend and intensity of the cranking viscosity changes as measured in the cold cranking simulator were thoroughly analysed. In the presented experiment, alterations in engine oils appearing during actual operation were noted. The tests were conducted under conditions which can be depicted as 'harsh', that is, multiplied starting of the engine, extended engine idling and short stretch driving. All of the engine oil samples were collected from passenger cars of a homogeneous fleet of 25 vehicles. The dynamic (cranking) viscosity was determined according to the ASTM D5293-15. In all analysed cases, there was a dangerously rapid increase (36%-69%) in the cranking viscosity, and the limit values (7000 mPa·s) were reached very quickly (for the mileages in the range of 3000-13,000 km). The obtained results have led to the development of a statistical model, allowing vehicle users/drivers to choose a better engine oil in winter, thus improving the engine's ability to cold start and protecting it from excessive wear or damage. The test results may help to predict the performance of the engine oil during operation, its service life and an oil-change interval.